This invention relates to a distillation process for the separation of CO.sub.2 from a gaseous mixture containing CO.sub.2 and light hydrocarbons as well as to an apparatus for conducting such a process.
In the distillation of light hydrocarbons, especially C.sub.1 to C.sub.6 hydrocarbons having a relatively high proportion of CO.sub.2, e.g., more than 5 molar %, a problem is encountered because of the propensity of CO.sub.2 to condense in solid form. This problem is particularly evident in the processing of CO.sub.2 -rich natural gases, i.e., natural gases having a CO.sub.2 content of more than about 5%, or in tertiary oil recovery where CO.sub.2 under high pressure is injected into oil bearing formations. In the latter case, besides petroleum, an accompanying gas containing light hydrocarbons is obtained which can contain, for example, between 5 and 95% CO.sub.2 ; usually, the CO.sub.2 content of this gas will gradually increase during the course of the tertiary oil recovery process from a relatively low value to a very high value while the amount of light hydrocarbons contained in the gas remains essentially constant. Whereas CO.sub.2 is generally separated from CO.sub.2 -rich natural gases because it is an undesirable impurity, in the case of tertiary oil recovery, CO.sub.2 is a desirable product stream which is reinjected into the formation under high pressure.
A conventional process for separating CO.sub.2 from light hydrocarbons provides for separation of a C.sub.1 fraction from the mixture in a first fractionating stage, and fractionation of the remaining C.sub.2+ --CO.sub.2 mixture in a further fractionating stage into CO.sub.2 and a C.sub.2+ fraction. However, this fractionation process is beset with a number of difficulties. Thus, when separating CH.sub.4 and CO.sub.2 under conditions normally prevailing during demethanization, solid CO.sub.2 deposits are formed in the fractionating column. In addition, during the subsequent separation of CO.sub.2 and C.sub.2+ hydrocarbons, CO.sub.2 forms an azeotrope with ethane at a CO.sub.2 /C.sub.2 molar ratio of about 2:1; consequently, further fractionation of this azeotrope by distillation requires the use of a special technique to change the relative volatilities of the CO.sub.2 and C.sub.2. Such a special technique is disclosed, for example in the so-called Ryan-Holmes process (Hydrocarbon Processing, May 1982, p. 131), wherein additives are introduced to prevent solid CO.sub.2 deposition or to break the CO.sub.2 /C.sub.2 azeotrope.
Unfortunately, the Ryan-Holmes process is excessively energy-intensive because in the fractionating stages, it is necessary to cool not only tne entire amount of gas to be fractionated to the respective operating temperatures in the two fractionating columns, but also the solvent additive as well. This leads to expensive cooling cycles and high operating costs, especially during the demethanization step which is conducted at relatively low temperatures (on the order of -80.degree. to -90.degree. C.).